1. Field of the Invention
The present invention relates to an apparatus for detecting a change in environment such as temperature, pressure, or the like and, more particularly, to a resistivity value measuring circuit having a detecting device whose resistivity value changes in dependence on the foregoing change.
2. Related Backqround Art
Hitherto, the operations of apparatuses which are used in the world (e.g., in the case of the thermal head, the heating operation of the head) are influenced by many environmental parameters such as temperature, humidity, pressure and the like. Therefore, it is necessary to perform the controls according to the values of those parameters. Recently, a microcomputer is frequently used to perform those controls. For this purpose, the environmental parameters need to be binarized to digital values and to input them into the microcomputer. On the other hand, as an environment detecting device, a device whose resistivity values change in response to changes in temperature, humidity, pressure, and the like is frequently used. Conventionally, as methods whereby the changes in those resistivity values are binarized and input into the microcomputer, the following methods have been known: a method as shown in FIG. 4 whereby a detecting device 21 such as a thermistor or the like is serially connected to a constant resistor 22 and connected to a constant voltage source, and a change in voltage at the connecting point is converted from a continuous value into a binary value by an A/D converter 23 and input; a method as shown in FIG. 5 whereby the detecting device 21 is arranged for a charge resistor of a monostable multivibrator 24 and the monostable multivibrator 24 is reset and the period of time when the output is generated is measured by a counter 25; and the like. However, these methods have drawbacks such that the cost is high, errors occur for a variation and fluctuation in devices and power source, and the like. Reference numeral 26 denotes a CPU and 27 is a linearity correciing resistor. The temperature-resisance characteristic of the thermistor 21 exponentially changes as shown by a curve A in FIG. 6. Therefore, a resistor is inserted in parallel or in series to the thermistor, thereby correcting so that the temperature-resistance characteristic becomes almost linear within a measurement range as shown by a straight line B in FIG. 6.